Combustion control



Oct. 26, 1965 J. P. WALKER ETAL COMBUSIION CONTROL Filed Oct. 50. 1962 ATTORNEY United States Patent 3,213,921 COMBUS'HON CONTROL Jay l. Walker and Jack A. Godwin, Tulsa, Okla., as-

signors to National Tank Company, Tulsa, Okla., a corporation of Nevada Filed Oct. 30, 1962, Ser. No. 234,098 11 Claims. (Cl. 158-127) The present invention relates to the control of cornbustion within a housing through which air passes to an industrial burner. The combustion is controlled to prevent its threat to nearby property and personnel. More particularly, the invention relates to a structure attached to the housing which will respond to a predetermined level of heat within the housing by positively stopping the passage of combustion air through the housing.

Surface equipment for producing oil and gas wells use industrial burners to supply heat for various purposes. Although the heat of combustion, when controlled and directed from the burner, can be very useful, uncontrolled combustion in and around the burner can become quite hazardous. If inammable hydrocarbon fluids leak into Zones in and around controlled combustion of the burner, there will be created a danger of widespread lire and eX- plosion.

It is common to provide a screen-like structure across the housing passage to prevent propagation of llashes of combustion from the burner side of the screen to an external source of combustible material. However, where there is a continual leaking of combustible material through these flash arrestor structures, along with combustion air, combustion on the burner side of the screen may take place for a period great enoug-h to bring the external temperature of the housing and/or the arrestor structure to the ignition temperature of the supply of eX- ternal combustible material. Therefore, there is a problem in detecting the propagation of combustion within the housing of a burner lthrough which the combustion air flows to the burner and control of this combustion to prevent reaching the ignition temperature of combustible material external the housing.

An object of the present invention is to detect the temperature of undesirable combustion within a housing which normally passes combustion air to an industrial burner and to positively stop the flow of air through the housing when the temperature reaches a predetermined value.

Another object is to positively shut off the normal flow of fuel to the burner at the same time the flow of air is stopped. Additionally, a visual and/or audible signal of the fuel stoppage is provided.

Another object is to detect the presence of undesirable combustion o-n the burner side of a flash arrestor structure mounted across a housing which normally conducts combustion air to the industrial burner and isolate air from the housing interior when the temperature of the combustion reaches a predetermined value.

The present invention contemplates a housing normally passing combustion air to an industria-l burner. An element is placed within the housing to physically elongate when heated by undesirable combustion in the housing. The movement of elongation by the detecting element is applied to a closure member for the entrance of air into the housing.

The invention further contemplates a valve in the normal supply of fuel to the burner actuated by the heated element or the closure member for air into the housing. Further, a switch is actuated at the same time as the valve to manifest the fact that the burner is no longer lit.

The invention further contemplates a combination of a flash arrestor structure for preventing propagation of cornbustion therethrough for relatively short periods of time and a structure which will detect unwanted and prolonged combustion on the burner side of the arrestor in order t0 positively seal off the entrance of all fluids, including air from the housing interior.

Other objects and advantages of the present invention will become readily apparent from the following detailed description of the invention with a specific reference to the accompanying drawing which is;

A sectioned elevation of a housing structure for the combustion air of an industrial burner in which is mounted a flash arresting screen-like structure and a temperature responsive element embodying the present invention.

The drawing depicts a housing 1, elongated along an axis. The housing, and the structure within and attached to the housing, is depicted in fairly complete section. Cornbustion air, and other fluids, llow into the housing 1, otherwise closed, through entrance 2. A burner 3 is mounted within the housing and supplied the air from entrance 2 and fuel through conduit 4. Flange 5 illustrates a means by which housing 1 is clamped to a vessel requiring the heat of combustion generated from burner 3.

Although housing 1 is considered as a unit, it is further illustrated as made up of three sections. Burner section 6 carries the ilange 5 and contains the burner 3 mounted therein. Directly behind section 6 is ash arrestor section 7, clamped between rear section 8 and the forwardburner section 6. These three sections are clamped together, in forming the housing 1, with joints which are mechanically rigid and fluid tight.

Joints Both joints are similar, so only one need be described. Joint 11 is formed by llanging burner section 6 -at 12 and flash arrest-or section 7 at 13. These flanges are brought together with a gasket material 14 between them.

The gasket may be selected from any number of suitable materials which will insure the fluid-tightness of the joint. Also, the material 14 must be resistant to a high degree of heat within housing 1.

A simple U-shaped hoop 15 is clamped about flanges 12 and 13 to draw them together, packing 14 positioned between them. With a solid surface backing up packing material 14, the pressure .of hoop 15 on the flanges 12 and 13 bring the entire joint into a simple, yet effective, seal.

loint 16 is similar to joint 11 and brings end housing portion 8 into junction with liash arrestor housing portion 7. Simple bolts, mounted through the lianges on the hoops of joints 11 and 16, provide ways of readily breaking down housing 1 into components for access to the structure mounted within housing 1. The structure within the housing then may be inspected, repaired or replaced with relative ease. In section 8, of housing 1, a simple, open-ended pipe 17 is mounted vertically in housing opening 18. The -cross-sectional shape and size of pipe 17 can be established as required to pass the cornbustion air in suicient'quantities into housing 1 to meet the demand on the industrial burner 3. Pipe 17 serves to illustrate that the invention includes the concept of a seat surface 19 which can be positively sealed against entry of combustion air to the interior of housing 1.

Seat 19 is sealed, specifically, by `engaging the seat with hinged lid 20. Lid 20 is pivoted clockwise about shaft 21, from the position illustrated, so lid 20 will meet seat 19 with a firm, positive, uniform contact. Lid 20 may be given any shape or weight required to effectively seal seat 19 against passage of fluids through pipe 17 and into housing 1.

In the drawing, lid 20 is shown as retained in an unseated position by retaining arm 22. When retaining arm 22 is moved a predetermined distance to the right, the arm will disengage from the upper edge of lid 20 and lid 20.will rotate clockwise, under the force of gravity, about pivot shaft 21 until seat 191s engaged and sealed.

The ultimate object of the invention is simple. All air, and combustible fluids, are to be positively sealed from the interior of housing 1 under certain conditions. When the seal is complete, all combustion within housing 1 and at the burner, will be snuffed out. The sealing of air from entry into the housing insures this control of undesirable combustion. The present invention provides a unique structure With which to actuate lid 20, as a closure member, to carry out this ultimate object of sealing.

Probe 25 Probe 25 is provided to actuate retaining arm 22 and release lid 20. In general, probe 25 is thrust into zone 26 where undesirable combustion is expected to take place within housing 1. Probe 25 responds to this undesirable combustion by elongating a component and applying this movement to retainer arm 22.

There could be several sources of undesirable combustion in zone 26. Inammable liquids and/ or gases could leak from the Vessel being heated by the industrial burner 3. Any external storage container of combustibles can fail and the material flow into Zone 26, along with air. In either of these events, the inflammables collecting within burner housing section 6 may be ignited from the controlled combustion of the industrial burner and develop an intense, and prolonged, heat at zone 26. Probe 25 responds to this combustion and acts through mechanical linkage to shift the position of retaining arm 22 to release lid 20. The result is a snufling of all combustion Within housing 1 because the combustion within the housing, whether from burner 3 or in zone 26, is starved ofy air.

Probe 25 is a generalized designation for a combination ofgparts lsupported within housing section 7. Probe housing 30 `is a cylindrical, elongated shell of material which will withstand the heat of prolonged combustion and which will elongate a desired amount as it heats. One end of shell 30 is mounted on nut 31 and extends into zone 26. The second end of shell 30 is fixed to one end of rod 32, While rod 32 extends back through shell 30. Rod 32 becomes the iirst element of a system of mechanical linkage between the free end of shell 30 which extends into zone 26 and retaining arm 22 which `holds lid 20 open. This rod 32 extends back through an axial bore of nut 31 and is attached to the longer end of link member 33 which is pivoted at 34 a short distance above this rpoint of attachment.

Multiplying linkage The multiplication of the axial movement of shell 30 is carried out by pivoting the upper, rst, end of a link -36 by attachment to the upper end 35 of link 33. Pivot point 37 is established for this upper end of link 36. IConnecting link 38 is then extended between the upper end 35 and point 39 which is on link 36, near pivot lpoint 37. This linkage arrangement represents several structures with which to establish a motion for the lower end 4t) of link 36 which is a magnification of the axial movement of rod 32 as rod 32 is moved by the elongation of shell 30.

It is almost obvious that rod 32 should expand and contract as little as possible as shell 30 moves under the thermal force of zone 26. Rod 32 lshould be made of material with a coefficient of expansion which is small, relative to that of shell 30. Rod 32 then becomes, essentially, a true mechanical link between the free end of shell 30 and the linkage system formed of links 33, 38 and 36. End 40 will then move a nite distance to the right, as viewed in the drawing, when the heat in zone 26 increases to a predetermined level.

There remains only to point out that retaining arm 22 is essentially a hook attached to the end 40 of link 36. Moved to the right, this arm 22 disengages from the edge 4 of lid 20. Gravity then acts on lid 2i) to pivot the lid about pivot shaft 21 until the underside of lid 20 meets seat 19 to stop the flow of air, and other fluids, across seat 19. Burner 3 is then starved of air; the combustion in zone 26 is starved of air; and all this combustion within housing 1 is snuifed out.

Flash arrestor All the structure within housing section 7 has heretofore been regarded as mechanical support for probe 25 and the linkage system to lid 20. However, although the Vstructure is arranged to give this support function to probe 25, ash arrestor 45 serves an interrelated function with the snuing mechani-sm.

Flash arrestorelement 45 is, in general, a screen-like structure with many passages through which iluids can readily pass from entrance 2 to burner 3. This screenlike element may take the form of a similar element disclosed in United States Patent 3,079,242 issued February 26, 1963 to Clarence O. Glasgow and assigned to National Tank Company. This screen-like element can be made in several sections or one section.

The basic function of arrestor element 45 is to provide passages which are so Small that mixtures at least as flammable as methane cannot propagate a iiame therethrough. The arrestor element can be made quite eilicient, from this standpoint, and is shown mounted transverse the longitudinal axis of housing 1, within section 7, to perform this function.

Arrestor element 45 is circular about the axis of housing 1. The outer edge ts snugly against the inner wall of housing Vsection 7 land is held in place by a framework including circular rims 46 and 47. These hoops, or rims, `46 and 47 are the outer elements of supporting and retaining frameworks on each side of flash arrestor element 45. The arrestor element 45 is thereby captured between rims 46 and 47 and sealed to the inner wall of -section 7 'by the body of therims. Fluids are thereby forced to ow through the holes of element 45 -in passage through housing 1.

T'he rims 46 and 47 extend parallel the housing axis, away from abutment with element 45 to back up packing in joints A11 and 16. Both rims `can be seen, in their Sections of the drawing, extending Ibeyond housing section 7. Rim 46 extends into housing section 1 and rim 47 extends into housing section 8. Fixed to housing section 7, rims 46 and 47 become mechanical guides for assembly of the three sections of housing 1. Also, with their outer surfaces beneath joints 11 and 16, the rims hold the packing of the joints 11 and 16 between flanges of the joints. (Refer to flanges 12 and 13 of joint 11.)

Referring to the center of element 45, probe 25 is disclosed, mounted as generally described supra. Nut 31 has been referred to, this nut supporting shell 30 and pa-ssing rod 32 for `actuating the linkage system.

Nut 31 is, itself, mounted in a central bore of pipe 48. This pipe 48 is a simple hollow tube, extended through the center of element 45. Nut 31 is mounted on the entrance 2 side of element 45, threaded into the 'bore of pipe 48. This nut 31 seals the bore of pipe 48 because packing is placed in its bore, about rod 32. With this arrangement, the probe 25 is supported to extend into zone 26 while fluid is forced to pass through the holes provided in element 45.

It is to be understood that probe 25 could be mounted at. other locations. Pipe 48 forms a convenient mount. However, the inventive concept includes other openings in element 45 as probe 25 can thereby lbe brought into a portion of zone 26 which will give quicker response to undesirable combustion. The concept remains around the elongation of a .probe element to positively disengage a structure that gravity may positively act on the structure in thereafter preventing air, and other fluids, from owing to a combustion process.

Fuel supply The invention has now been shown as capable of controlling the flow of fluids to combustion within housing 1, the fluids heretofore regarded as coming through entrance 2. However, the concept of control also includes the regulation of fuel normally flowing to burner 3 through conduit 4.

Burner 3 also has a pilot burner 49. The valving and conduits for these burners can be a complex system. IFor disclosing the present invention, attention is coniined to valve 50 in conduit 4. Valve 50 is placed so its closing will positively and completely stop the flow of fuel to burner 3 and its associated pilot burner 49. Valve 50 becomes the means through which all fuel normally owing t-o burner structure within housing 1 is halted when the flow of fluids through entrance 2 is stopped by lid 20.

Lid is brought into the combination, actuating valve 50 at the proper time. A small pilot valve 51 is attached to conduit 17 and its lactuating stem 52 placed so it will be contacted by lid 20 as lid 20 pivots into sealing position on seat 19. Valve 51 controls actuating fluid to the fuel valve 50. Therefore, when valve 51 is actuated by lid 20, fuel valve 50 is closed.

It is apparent that any number of signaling structures can lbe actuated at the same time fuel is shut off to burner 3. These signaling devices can use lights to attract the attention of operating personnel. Horns or bells can also be used. The informed pumper then knows to go to the lease where the combustion has been lsnutfed out and determine the cause of the oper-ation of the system. Once burner 3 is returned to normal operation, actuating arm 22 can be manually re-engaged with lid 20 by the pumper reaching his hand up through entrance 2. Arm 22 can be made flexible enough to re-hook lid 20 or a simple latch can be provided to enable arm 22 to move back under lid 26 as shown in the drawing.

Protec-t ve screen:

The recocking of lid 20 brings into focus the vulner- Iability of any linkage system to dirt, debris, bugs and other material which can enter housing 1 through pipe 17 and accumulate on the linkage. These accumulations on the mechanism within housing portion i8 can jam and prevent the linkage from operating normally as intended.

Screen S5 is mounted across entrance 2 to prevent entry of rnuch of the material which would dirty the linkage and prevent its normal function. Of course, the mesh of the screen 55 must be large enough to not reduce below satisfactory limits the ow of air required for normal 'burner combustion. An adequate supply of air :during normal combustion is a big problem. The arrestor element introduces some pressure drop in the air iiow across it. The entrance pipe causes an additional drop. Now the bug screen 55 can become still yanother factor in obstructing the ow of air through housing 1. The output of burner 3 becomes subject to these factors and their sizes must lbe 'both calculated and empirically adjusted to provide the necessary amount of air to the burner.

Linkage housing Still another structure is provided to isolate debris from the multiplying linkage system. A box 56 has one side shown in the drawing. 'Ihis box cove-rs the linkage 'well enough to lkeep it clean and free of 'any material which does iind its way into housing section 8. The housing may take various forms, but its function is always -to keep the linkage system clean while .permitting it lfreedom to function as intended.

Box I56 may he mounted in various ways. [It is indicated ias xed to nut 31. Rather, nut 31 is indicated as extending through a wall of lthe fbox to thread int-o the 'bore of pipe 48. Any mounting that supports .the box 56 in the position indicated will `be satisfactory. 'Ihe basic requirement is that this cover keep the linkage clean and operative.

From the foregoing it will -be 'seen that this invention is one well adapted to attain all of the tends .and objects hereinabove set forth, together with other advantages which are obvious and `which are inherent to `the Iapparatas.

-It will be understood that certain lfeatures and subcombinations :are of utility and may be employed without reference to other features and subcombinations. This is contemplated fby and is within the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be -understood that all matter herein set forth or shown in the accompanying drawings is .to be interpreted as i-lllustnative and not in a limiting sense.

The present invention having been described, what is claimed is:

1. A combustion control system including,

a :burner adapted t-o propagate Icombustion at a predetermined location,

a closed housing `through which air for the support of combustion by the burner passes,

an opening in the housing for entry of the air,

a closure ymember for the opening to stop the passage of air through the opening and into the housing,

means positioned in the housing -tohold the closure member away from the opening,

means positioned in the housing to respond to combustion in the housing, other than the combustion propa- :gated by the fburner at the predetermined location, by actuating the holding means for the closure mem- Iber to release .the closure member `which .then shuts the opening and prevents air from reaching any combustion propagated by the 'burner and in the housing.

2. A combustion control system including,

a closed housing conducting air to an industrial burner which is yadapted to propagate combustion 4at `a predetermined location,

a single opening through the housing wall for entry of air into the housing,

a closure mem-ber -for the opening mechanical-ly actuated to sea-l the opening -against entry of air,

means positioned in the housing to sense the temperature of combustion in the housing, other than the vcombustion propagated `by the burner at the predetermined location, and establishing la mechanical motion representative Iof the temperature,

`and a mechanical linkage extending between the temperature sensing means and the closure member, lwhereby the lclosure member is caused to seal the housin-g opening when the temperature of the combustion other than the combustion pnopagated by the burner reaches a predetermined level.

3. A combustion control system including,

a closed housing conducting air to van industrial burner which is adapted to propagate combustion at a predetermined location,

a single opening through the housing wa-ll for entry of air into the housing,

a lid hinged near the opening t-o fall by gravity on the opening and seal the opening against entry of air,

means positioned fin the housing to sense the temperatune of combustion in the housing, other than t-he combustion propagated -by the burner at the predetermined location, and establishing a mechanical tm-otion representative -of the temperature,

and a mechanical linka-ge connecting the temperature sensing means to `the lid to hold the lid hinged away from lthe opening and release the lid when the temperature of the combustion, other than the combustion propagated by the hurner, reaches la predeteramined value so that the lid will fall by gravity on the opening and seal the opening.

4. A combustion control system including,

closed housing conducting air to an industrial burner Which is adapted to propagate combustion at a predetermined location, single lopening through the housing Wall for entry of air into the housing,

lid hinged near the opening to fall by gravity on the opening and seal the opening against entry of air,

an elongated member mounted by one end to the housi-ng and extending into -a housing area in which it is desired to sense the temperature of combustion in the housing, other than the combustion propagated by the burner at the predetermined location, the member changing dimension as the temperature of the combustion chan-ges to move its other end,

:and a lmechanical linkage connecting the other end of the elongated member to the lid to hold the lid hinged away from the opening and release the lid when the temperature of the combustion, other than the combustion propagated by the burner, reaches a predetermined value so that the lid will tall by gravity on the opening and seal the opening.

A combustion control system including,

burner requiring air to support combustion at ra predetermined location,

closed housing conducting air to the burner at .the predetermined location,

an entry in the housing for tluids passing through the housing,

a lid which can seal the entry,

screen-'like barrier mounted in the housing between the entry and burner which prevents combustion being propagated between its 'burner side and its housing entry side for relatively short periods of time,

and means mounted in the housing to respond to combusti-on, other than the combustion propagated `by the burner, on the barrier side of the burner to position the lid over the entry to positively stop air from passing through the housing when the combustion, other than the combustion propagated by the burner, within the housing on the burner side of the barrier reaches a predetermined level.

A combustion control sy-stem including,

closed housing conducting air to an industrial burner which is adapted to prop-agate combustion at a predetermined location, single opening through the housin-g Wall for entry of air into the housing,

closure member for the opening mechanically actu- I ated to seal the opening against entry of air,

a barrier mounted in the housing between the air entry and a mechanical linkage extending between the temperature sensing means .and the closure member, whereby the closure member is caused to -seal the housing opening when the temperature of the uridesinabfle combustion, othert-han that propagated by the burner, within the housing reaches a predetermined level.

A combustion control system including,

a closed housing conducting air to an industrial burner 8 which is adapted to propagate combustion at a predetermined location, single opening through the housing wall for entry of air into the housing, lid hinged near the opening to fall by gravity on the opening and seal the opening against entry of air, barrier mounted in the housing between the air entry opening and the exit of the housing, the barrier permitting the passage of lluids while functioning eii'ectively to prevent propagation of combustion on the side of the barrier toward the housing exit back to the side of the barrier toward the entry opening of the housing for limited periods of time,

a means mounted in the housing between the exit opening of the housing and the barrier to respond to the temperature of combustion, other than that combustion propagated by the burner Within the housing, and establish a mechanical motion representative of the temperature,

and a mechanical linkage connecting the temperature sensing means to the lid to hold the lid'hinged away from the opening and release the lid when the temperature of the combustion, other than that combustion propagated by the burner, reaches a predetermined value so ythat the lid Will fall by gravity on the opening and seal the opening.

A combustion control system including,

closed housing conducting air to an industrial burner which is adapted to propagate combustion at a predetermined location,

single opening through the housing Wall for entry of air into the housing,

lid hinged near the opening to fall by gravity on the opening and seal the opening a-gainst entry of air, barrier mounted in the housing between the air entry opening and the exit of the housing, the `barrier Permitting the passage of fluids While functioning eiectively to prevent propagation of combustion within the housing and on the side of the barrier toward the housing exit back to the side of the barrier toward the entry opening of the housing for limited periods of time,

an elongated member mountedby one end to the housing and extending into the housing between the exit opening of the housing and the barrier to sense the temperature of combustion, other than that combustion propagated by the burner within the housing, the member changing dimension as the temperature of combustion changes to move the other end,

and a mechanical linkage connecting the other end of the elongated member to the lid to hold the lid hinged away from the opening and release the lid when the temperature of the combustion, other than that combustion propagated by the burner, reaches a predetermined value so that the lid will fall by gravity on the opening and seal the opening.

A combustion control system including,

closed housing having a cylindrical section through which air is conducted to an industrial burner which is adapted to propagate combustion at a predetermined location,

single opeuing through a horizontal wall of the houS- ing for entry of the air into the cylindrical section, lid hin-ged near the opening so the lid will gravitate down on the opening and seal the opening against entry of air,

screen-like barrier mounted in the cylindrical section of the housing to extend transverse the axis of the section and providing holes therethrough which are small enough to prevent the propagation of combustion from one side of the barrier to the other,

a rod mounted at the center of the barrierand extending into a housing area between the barrier and the exit for the air to transmit a motion back through the barrier to the side having the entry opening through the housing Wall in accordance with the temperature of combustion, other than that combustion propagated by the burner,

and a mechanical linkage connecting the rod to the lid, whereby the lid is held from falling on the housing opening and sealing the opening until the temperature of the combustion, other than that combustion propagated by the burner, reaches a predetermined value.

10. A combustion control system including,

a burner at which combustion is propagated at a predetermined location,

a closed housing arranged to conduct all air supporting combustion at the burner,

a seat at the entrance of the housing across which the air passes into the housing,

a lid arranged to pivot into sealing engagement with the seat under the force of gravity,

a structural member arranged in a zone of combustion Within the housing and engaging the lid to hold the lid pivoted away from the seat until the combustion, other than the combustion propagated by the burner, elongates the member and causes the member to disengage the lid,

a conduit supplying fuel normally to the burner,

10 and a valve in the fuel conduit arranged to be actuated when the lid is disengaged from the structural member to shut off the fuel to the burner. 11. The system of claim 10, including, a means to signal closure of the seat and shut off of burner fuel.

References Cited by the Examiner UNITED STATES PATENTS 2,636 5/42 Foote 158-28 1,295,856 3/19 Couture 15S-42.4 1,377,873 5/21 Chadick 158-424 1,773,198 8/30 Piatt.

1,862,630 6/32 Morrow 236-1 1,963,995 6/34 Journey 158-91 2,010,305 8/35 Latimer 126-2875 X 2,137,199 11/38 Betz 158-124 2,194,713 3 40 Mitchell. 3,124,194 3/64 Fuchs 15S-42.4

FOREIGN PATENTS 174,870 4/6-1 Sweden.

JAMES W. WESTHAVER, Primary Examiner.

25 MEYER PERLIN, Examiner. 

1. A COMBUSTION CONTROL SYSTEM INCLUDING, A BURNER ADAPTED TO PROPAGATE COMBUSTION AT A PREDETERMINED LOCATION, A CLOSED HOUSING THROUGH WHICH AIR FOR THE SUPPORT OF COMBUSTION BY THE BURNER PASSES, AN OPENING IN THE HOUSING FRO ENTRY OF THE AIR, A CLOSURE MEMBER FOR THE OPENING TO STOP THE PASSAGE OF AIR THROUGH THE OPENING AND INTO THE HOUSING, MEANS POSITIONED IN THE HOUSING TO HOLD THE CLOSURE MEANS AWAY FROM THE OPENING, MEANS POSITIONED IN THE HOUSING TO RESPOND TO COMBUSTION IN THE HOUSING, OTHER THAN THE COMBUSTION PROPAGATED BY THE BURNER AT THE PREDETERMINED LOCATION, BY ACTUATING THE HOLDING MEANS FOR THE CLOSURE MEMBER TO RELEASE THE CLOSURE MEMBER WHICH THEN SHUTS THE OPENING AND PREVENTS AIR FROM REACHING ANY COMBUSTION PROPAGATED BY THE BURNER AND IN THE HOUSING. 